RNA interference or “RNAi” is a term initially coined by Fire and co-workers to describe the observation that double-stranded RNA (dsRNA) can block gene expression when it is introduced into worms (Fire et al., Nature 391:806-811, 1998). Short dsRNA directs gene-specific, post-transcriptional silencing in many organisms, including vertebrates, and has provided a new tool for studying gene function.
Numerous myelin-derived axon growth inhibitors have been characterized (see, for review, David et al., WO995394547, 1999; Bandman et al. U.S. Pat. No. 5,858,708, 1999; Schwab, Neurochem. Res. 21:755-761, 1996). Several components of CNS white matter, NI35, NI250 (Nogo) and Myelin-associated glycoprotein (MAG), which have inhibitory activity for axonal extension, have been described as well (Schwab et al., WO9005191, 1990; Schwab et al., U.S. Pat. No. 5,684,133, 1997). In particular, RhoA is a member of the large family of Rho (Ras homologue) GTPases, itself belonging to the superfamily of Ras GTPases. All eukaryotes contain at least one Rho GTPase. During the process of evolution the number of Rho GTPases increased from 5 to 6 per organism (yeast) to over 20 (mammals) (Karnoub, A. E., et al., Breast Cancer Res. Treat. 2004, 84:61). Like other GTPases, RhoA has intrinsic GTPase activity and shuttles between an inactive GDP-bound state and an active GTP-bound state. In vitro, the exchange of GDP to GTP occurs very slowly, and is catalyzed by guanine nucleotide exchange factors (GEFs), which exchange GDP for GTP. GTPase activating proteins (GAPs) catalyze hydrolysis of the γ-phosphate of GTP. (Wheeler, A. P., Ridley, A. J., Exp. Cell Res. 2004, 301:43). A third set of regulatory proteins, the guanine nucleotide-dissociation inhibitors (GDIs), sequester GTPAses in the cytosol in the inactive, GDP-bound state.
The N-terminal half of Rho GTPases contains the majority of the amino acids involved in GTP binding and hydrolysis, together with the Switch 1 and 2 regions that change conformation between the GTP-bound and GDP-bound states (Bishop, A. L., Hall, A., Biochem. J. 2000, 348 (Pt. 2):241). The C-terminus of Rho family GTPases is essential for correct localization of the proteins. It is post-translationally modified by prenylation of a conserved C-terminal cysteine followed by methylation and proteolytic removal of the last three amino acids (Shao, F., Dixon, J. E., Adv. Exp. Med. Biol. 2003, 529:79). The prenyl group anchors the GTPases into membranes and this modification is essential for cell growth, transformation, and cytoskeleton organization (Allal, C., et al., J. Biol. Chem. 2000, 275:31001). Prenylation of Rho proteins appears to be important for their stability, inhibitors of enzymes that synthesize prenyl groups induce a decrease in Rho protein levels and their function (Stamatakis, K., et al., J. Biol. Chem 2002, 277:49389). In the case of RhoA, prenylation adds a geranylgeranyl group. RhoA is mainly found in the cytoplasm or at the plasma membrane (Adamson, P., et al., J. Cell Biol. 1992, 119:617).
RhoA may bind to the intracellular portion of p75NTR and is activated by Nogo-R in a p75NTR-dependent manner (Wang, K. C., et al., Nature 2002, 420:74), which is how MAG, Nogo-66, and oligodendrocyte-myelin glycoprotein achieve RhoA activation. The central inhibitory domain of Nogo-A, NiG, distinct from Nogo-66, and Versican V2, a chondroitin-sulfate proteoglycan and another component of myelin, are able to activate RhoA in the absence of p75NTR, by an alternative pathway of RhoA activation remaining to be elucidated (Schweigreiter, R., et al., Mol. Cell Neurosci. 2004, 27:163). Further pathways of activation may exist.
RhoA is part of the growth inhibitory machinery present in the central nervous system (CNS), but not in peripheral nerves, which prevents the regeneration of CNS tissue after injury. Both the expression and the activation of RhoA is induced in brain and spinal cord injury (Mueller, K., et al., Nature Reviews 2005, 4:387). Activation of RhoA leads to neuronal growth cone collapse, retraction bulb formation and neurite withdrawal. Inactivation of RhoA leads to neurite outgrowth in primary neurons on otherwise inhibitory substrates in vitro, and promotes axon regeneration and functional recovery after spinal cord injury in rats and mice in vivo (Lehmann, M. A., et al., J. Neurosci. 1999, 19:7537; Hara, M, et al., J. Neurosurg. 2000, 93:94; Dergham, P., et al., J. Neurosci. 2002, 22:6570). Furthermore, inactivation of Rho has been shown to protect endogenous cells of the spinal cord from apoptosis induced by spinal cord injury (Dubreuil, C. I., et al, J. Cell Biol. 2003, 162:233). These findings have clinical relevance because neuroprotective treatments after spinal cord injury lead to improved functional recovery (Liu, X. Z., et al., J. Neurosci. 1997, 17:5395).
Evidently, RhoA is a potential target for therapeutic intervention strategies aimed at diseases and conditions involving, e.g., the destruction and/or impaired regeneration of cells of the CNS. The present invention advances the art by providing methods and medicaments encompassing short dsRNAs leading to the downregulation of RhoA mRNA and protein levels in cells expressing the RhoA gene. These methods and medicaments may be used in the treatment of disorders or pathological processes mediated, at least in part, by RhoA, e.g., by preventing the RhoA inhibition of axonal elongation and regeneration, and consequently stimulating nerve growth and proliferation.